1. Field of the Invention
The present invention relates to the field of solid state laser drivers.
2. Prior Art
Laser drivers of various designs are well known in the prior art. Such devices are used to drive solid state lasers in optical transmitters for optical transmission of data over fiber optic lines. These devices are capable of high data rates, and accordingly, are finding ever increasing application in data communications. Solid state lasers, however, have various characteristics that must be accounted for in such applications. One such characteristic is that, at least for high data rates, solid state lasers are not operated between on and off conditions because of the time it takes for the laser, when fully off, to get back into a lasing condition. Accordingly, for high data rates, such lasers are commonly operated between a high optical power level as one data state and a much lower power level for the second data state. For purposes herein, it will be assumed that the high optical power level will represent a logic 1 and the low optical power level will represent a logic 0. The ratio of optical power levels, P1 for the high optical power level and P0 for the low optical power level, is referred to as the extinction ratio (P1/P0). Normally the user of the laser driver, i.e., the manufacturer of the optical transmitter, desires to set and control the extinction ratio and the average power. Furthermore the laser driver outputs a DC current Ibias ensuring laser operation in lasing mode and a modulated AC current Imod defining the logic 1 optical power level and the logic 0 optical power level.
There are two general types of optical transmitters: continuous and time-division multiplexed, such as a passive optical network (PON). The PON system requires a control signal, burst enable (BEN), which manages multiple transmitters on the same fiber sharing the same wavelength by only enabling one transmitter at a time. Burst-mode operation adds complexity to the control loop operation due to the lack of data while BEN is low for the particular transmitter. The control loop must have the ability to either converge during the burst-on time or save the loop state between bursts. Since the minimum burst-on time is on the order of hundreds of nanoseconds and the typical loop time constant is at least an order of magnitude greater, converging during the burst-on time is normally not feasible. Ideally, the loop state should be kept in a dynamic freeze mode, where the digital states (e.g., values I0, I1, Ibias, Imod, as described later in this document) are frozen, but the analog signals (e.g., the input signals to Filterin and Filterref of the present invention as described later in this document) are preferred to be active to retain the Vin and Vref of the present invention at the last known operating point either using the current data input or an approximation of normal data if normal data is not available.
Optical transmitters typically each include a monitor diode which receives a part of the light emitted by the transmitting diode to provide a measure of the optical power levels of the transmitting laser diode. However such monitor diodes and their associated circuitry do not have the high frequency capabilities of the transmitting diode, and accordingly, have real limitations with respect to what the monitor diode can accomplish. In particular, the monitor diode can easily sense the average power (P1+P0)/2 as very little bandwidth is needed to do so, but in general cannot sense either P1 or P0 unless sufficient consecutive identical digits (CID) are first applied which represents the transmission of all ones for sensing P1 and transmission of all zeroes for sensing P0. While this can be done, it has the disadvantage of requiring the same to be done, typically periodically, so that changes in temperature do not let either value get out of control. This of course provides an undesired interruption of data transmission. Alternatively, a CID detector may be implemented which enables the sensing for P1 or P0 when a sufficient number of CIDs are detected in the data stream. This is costly to implement and may result in too few updates to reliably track the laser behavior. The timely occurrence of such a CID data stream is non deterministic due to the random nature of the data being transmitted. Systems which require periodic occurrence of CIDs are not practical.
Another approach that has been used is to superimpose a relatively low frequency on either the bias current Ibias or on the modulating current Imod, or both, to detect the slope of the optical power curve. The detected laser slope, together with a target extinction ratio, a target average optical power and a measure of the then operating average optical power, can be used to control the values of Ibias and Imod to obtain the desired values of P0 and P1. The problem, however, is that the optical power versus transmitting diode current is not linear, so that applying a fixed slope is, at best, a relatively rough approximation of the actual laser behavior. In that regard, a reasonable error in the value of P1 can be tolerated. However the same magnitude of error in the value of P0 cannot be tolerated because P0 is typically such a small value anyway. The same error in P0 can cause a very large change in the extinction ratio, and may result in an Ibias value below the laser threshold, causing laser relaxation oscillations and unacceptable transmitting laser diode performance.
FIG. 1 is a copy of FIG. 6 from U.S. Patent Application Publication No. 2002/0027690, illustrating a still different way of controlling P1 and P0. In this Figure the outputs on lines 120 and 122 are laser bias control and laser modulation control inputs to the laser driver. In this Figure the current in line 117 from the monitor diode is converted to a voltage by transimpedance amplifier 200, with peak detector 204 sensing the peaks in the monitor diode output and the valley detector 206 sensing the lowest values of the output of the monitor diode. These peaks and valleys are taken as a measure of P1 and P0 which, after processing, provide the laser modulation and laser bias control signals. While this system provides closed loop control, it is highly dependent on the data pattern being transmitted, and depends on the data containing significant strings of all ones and all zeros within a sampling time. It is also dependent on the DC offsets in each control path, which can be substantial, particularly in comparison to the value of P0. U.S. Pat. No. 5,974,063 is similar to this system in some respects.
Other examples of prior art techniques may be found in U.S. Pat. Nos. 5,502,298, 5,535,038, 5,850,409, 6,414,974, 6,807,209, 6,829,267, 6,859,473, 6,907,055, 6,928,094, 6,993,459, 7,088,752, 7,142,574, 7,245,828 and 7,349,454 and U.S. Patent Application Publication Nos. 2005/0226292 and 2009/0310961. Also, equipment for measuring or sensing power levels is also commercially available, such as Agilent Digital Communication Analyzer and Optical Sampling Oscilloscope (86100A/B/C, 86106B, 86107A, 86119A) and Agilent Multi-Channel Power Meter (N7751A, N7752A, N7761A, N7762A, N7764A).